Japanese natural stones are traditionally used in Japan to sharpen all kinds of knives and metal tools. These stones are mostly mined from the mountains around Kyoto. Depends on their size, purity, appearance, speed and fineness, the natural stones can be extremely expensive. They are still often used in katana sharpening. Because only natural stones can create the elusive haze finish on the knives, the good ones are highly thoughafter by professionals and collectors.